The fore-brain or prosencephalon consists of: (1) the diencephalon, corresponding in a large measure to the third ventricle and the structures which bound it; and (2) the telencephalon, comprising the largest part of the brain, viz., the cerebral hemispheres; these hemispheres are intimately connected with each other across the middle line, and each contains a large cavity, named the lateral ventricle. The lateral ventricles communicate through the interventricular foramen with the third ventricle, but are separated from each other by a medial septum, the septum pellucidum; this contains a slit-like cavity, which does not communicate with the ventricles.

The Diencephalon.The diencephalon is connected above and in front with the cerebral hemispheres; behind with the mid-brain. Its upper surface is concealed by the corpus callosum, and is covered by a fold of pia mater, named the tela chorioidea of the third ventricle; inferiorly it reaches to the base of the brain.

The diencephalon comprises: (1) the thalamencephalon; (2) the pars mamillaris hypothalami; and (3) the posterior part of the third ventricle. For descriptive purposes, however, it is more convenient to consider the whole of the third ventricle and its boundaries together; this necessitates the inclusion, under this heading, of the pars optica hypothalami and the corresponding part of the third ventriclestructures which properly belong to the telencephalon.

The Thalamencephalon.The thalamencephalon comprises: (1) the thalamus; (2) the metathalamus or corpora geniculata; and (3) the epithalamus, consisting of the trigonum habenulæ, the pineal body, and the posterior commissure.

The Thalami (optic thalamus) (Figs. 716,717) are two large ovoid masses, situated one on either side of the third ventricle and reaching for some distance behind that cavity. Each measures about 4 cm. in length, and presents two extremities, an anterior and a posterior, and four surfaces, superior, inferior, medial, and lateral.

The posterior extremity is expanded, directed backward and lateralward, and overlaps the superior colliculus. Medially it presents an angular prominence, the pulvinar, which is continued laterally into an oval swelling, the lateral geniculate body, while beneath the pulvinar, but separated from it by the superior brachium, is a second oval swelling, the medial geniculate body.

The superior surface is free, slightly convex, and covered by a layer of white substance, termed the stratum zonale. It is separated laterally from the caudate nucleus by a white band, the stria terminalis, and by the terminal vein. It is divided into a medial and a lateral portion by an oblique shallow furrow which runs from behind forward and medialward and corresponds with the lateral margin of the fornix; the lateral part forms a portion of the floor of the lateral ventricle, and is covered by the epithelial lining of this cavity; the medial part is covered by the tela chorioidea of the third ventricle, and is destitute of an epithelial covering. In front, the superior is separated from the medial surface by a salient margin, the tænia thalami, along which the epithelial lining of the third ventricle is reflected on to the under surface of the tela chorioidea. Behind, it is limited medially by a groove, the sulcus habenulæ, which intervenes between it and a small triangular area, termed the trigonum habenulæ.

The medial surface constitutes the upper part of the lateral wall of the third ventricle, and is connected to the corresponding surface of the opposite thalamus by a flattened gray band, the massa intermedia (middle or gray commissure). This mass averages about 1 cm. in its antero-posterior diameter: it sometimes consists of two parts and occasionally is absent. It contains nerve cells and nerve fibers; a few of the latter may cross the middle line, but most of them pass toward the middle line and then curve lateralward on the same side.

The lateral surface is in contact with a thick band of white substance which forms the occipital part of the internal capsule and separates the thalamus from the lentiform nucleus of the corpus striatum.

Structure.The thalamus consists chiefly of gray substance, but its upper surface is covered by a layer of white substance, named the stratum zonale, and its lateral surface by a similar layer termed the lateral medullary lamina. Its gray substance is incompletely subdivided into three partsanterior, medial, and lateralby a white layer, the medial medullary lamina. The anterior part comprises the anterior tubercle, the medial part lies next the lateral wall of the third ventricle while the lateral and largest part is interposed between the medullary laminæ and includes the pulvinar. The lateral part is traversed by numerous fibers which radiate from the thalamus into the internal capsule, and pass through the latter to the cerebral cortex. These three parts are built up of numerous nuclei, the connections of many of which are imperfectly known.

Connections.The thalamus may be regarded as a large ganglionic mass in which the ascending tracts of the tegmentum and a considerable proportion of the fibers of the optic tract end, and from the cells of which numerous fibers (thalamocortical) take origin, and radiate to almost every part of the cerebral cortex. The lemniscus, together with the other longitudinal strands of the tegmentum, enters its ventral part: the thalamomammillary fasciculus (bundle of Vicq dAzyr), from the corpus mammillare, enters in its anterior tubercle, while many of the fibers of the optic tract terminate in its posterior end. The thalamus also receives numerous fibers (corticothalamic) from the cells of the cerebral cortex. The fibers that arise from the cells of the thalamus form four principal groups or stalks: (a) those of the anterior stalk pass through the frontal part of the internal capsule to the frontal lobe; (b) the fibers of the posterior stalk (optic radiations) arise in the pulvinar and are conveyed through the occipital part of the internal capsule to the occipital lobe; (c) the fibers of the inferior stalk leave the under and medial surfaces of the thalamus, and pass beneath the lentiform nucleus to the temporal lobe and insula; (d) those of the parietal stalk pass from the lateral nucleus of the thalamus to the parietal lobe. Fibers also extend from the thalamus into the corpus striatumthose destined for the caudate nucleus leave the lateral surface, and those for the lentiform nucleus, the inferior surface of the thalamus.

The medial geniculate body (corpus geniculatum mediale; internal geniculate body; postgeniculatum) lies under cover of the pulvinar of the thalamus and on the lateral aspect of the corpora quadrigemina. Oval in shape, with its long axis directed forward and lateralward, it is lighter in color and smaller in size than the lateral. The inferior brachium from the inferior colliculus disappears under cover of it while from its lateral extremity a strand of fibers passes to join the optic tract. Entering it are many acoustic fibers from the lateral lemniscus. The medial geniculate bodies are connected with one another by the commissure of Gudden, which passes through the posterior part of the optic chiasma.

The lateral geniculate body (corpus geniculatum laterale; external geniculate body; pregeniculatum) is an oval elevation on the lateral part of the posterior end of the thalamus, and is connected with the superior colliculus by the superior brachium. It is of a dark color, and presents a laminated arrangement consisting of alternate layers of gray and white substance. It receives numerous fibers from the optic tract, while other fibers of this tract pass over or through it into the pulvinar. Its cells are large and pigmented; their axons pass to the visual area in the occipital part of the cerebral cortex.

The superior colliculus, the pulvinar, and the lateral geniculate body receive many fibers from the optic tracts, and are therefore intimately connected with sight, constituting what are termed the lower visual centers. Extirpation of the eyes in a newly born animal entails an arrest of the development of these centers, but has no effect on the medial geniculate bodies or on the inferior colliculi. Moreover, the latter are well-developed in the mole, an animal in which the superior colliculi are rudimentary.

The trigonum habenulæ is a small depressed triangular area situated in front of the superior colliculus and on the lateral aspect of the posterior part of the tænia thalami. It contains a group of nerve cells termed the ganglion habenulæ. Fibers enter it from the stalk of the pineal body, and others, forming what is termed the habenular commissure, pass across the middle line to the corresponding ganglion of the opposite side. Most of its fibers are, however, directed downward and form a bundle, the fasciculus retroflexus of Meynert, which passes medial to the red nucleus, and, after decussating with the corresponding fasciculus of the opposite side, ends in the interpeduncular ganglion.

The pineal body (corpus pineale; epiphysis) is a small, conical, reddish-gray body which lies in the depression between the superior colliculi. It is placed beneath the splenium of the corpus callosum, but is separated from this by the tela chorioidea of the third ventricle, the lower layer of which envelops it. It measures about 8 mm. in length, and its base, directed forward, is attached by a stalk or peduncle of white substance. The stalk of the pineal body divides anteriorly into two laminæ, a dorsal and a ventral, separated from one another by the pineal recess of the third ventricle. The ventral lamina is continuous with the posterior commissure; the dorsal lamina is continuous with the habenular commissure and divides into two strands the medullary striæ, which run forward, one on either side, along the junction of the medial and upper surfaces of the thalamus to blend in front with the columns of the fornix.

The posterior commissure is a rounded band of white fibers crossing the middle line on the dorsal aspect of the upper end of the cerebral aqueduct. Its fibers acquire their medullary sheaths early, but their connections have not been definitely determined. Most of them have their origin in a nucleus, the nucleus of the posterior commissure (nucleus of Darkschewitsch), which lies in the central gray substance of the upper end of the cerebral aqueduct, in front of the nucleus of the oculomotor nerve. Some are probably derived from the posterior part of the thalamus and from the superior colliculus, while others are believed to be continued downward into the medial longitudinal fasciculus.

The Hypothalamus(Fig. 720) includes the subthalamic tegmental region and the structures forming the greater part of the floor of the third ventricle, viz., the corpora mammillaria, tuber cinereum, infundibulum, hypophysis, and optic chiasma.

The subthalamic tegmental region consists of the upward continuation of the tegmentum; it lies on the ventro-lateral aspect of the thalamus and separates it from the fibers of the internal capsule. The red nucleus and the substantia nigra are prolonged into its lower part; in front it is continuous with the substantia innominata of Meynert, medially with the gray substance of the floor of the third ventricle.

It consists from above downward of three strata: (1) stratum dorsale, directly applied to the under surface of the thalamus and consisting of fine longitudinal fibers; (2) zona incerta, a continuation forward of the formatio reticularis of the tegmentum; and (3) the corpus subthalamicum (nucleus of Luys), a brownish mass presenting a lenticular shape on transverse section, and situated on the dorsal aspect of the fibers of the base of the cerebral peduncle; it is encapsuled by a lamina of nerve fibers and contains numerous medium-sized nerve cells, the connections of which are as yet not fully determined.

The corpora mammillaria (corpus albicantia) are two round white masses, each about the size of a small pea, placed side by side below the gray substance of the floor of the third ventricle in front of the posterior perforated substance. They consist of white substance externally and of gray substance internally, the cells of the latter forming two nuclei, a medial of smaller and a lateral of larger cells. The white substance is mainly formed by the fibers of the columns of the fornix, which descend to the base of the brain and end partly in the corpora mammillaria. From the cells of the gray substance of each mammillary body two fasciculi arise: one, the thalamomammillary fasciculus (bundle of Vicq dAzyr), passes upward into the anterior nucleus of the thalamus; the other is directed downward into the tegmentum. Afferent fibers are believed to reach the corpus mammillare from the medial lemniscus and from the tegmentum.

FIG. 720 Median sagittal section of brain. The relations of the pia mater are indicated by the red color. (See enlarged image)

The tuber cinereum is a hollow eminence of gray substance situated between the corpora mammillaria behind, and the optic chiasma in front. Laterally it is continuous with the anterior perforated substances and anteriorly with a thin lamina, the lamina terminalis. From the under surface of the tuber cinereum a hollow conical process, the infundibulum, projects downward and forward and is attached to the posterior lobe of the hypophysis.

In the lateral part of the tuber cinereum is a nucleus of nerve cells, the basal optic nucleus of Meynert, while close to the cavity of the third ventricle are three additional nuclei. Between the tuber cinereum and the corpora mammillaria a small elevation, with a corresponding depression in the third ventricle, is sometimes seen. Retzius has named it the eminentia saccularis, and regards it as a representative of the saccus vasculosus found in this situation in some of the lower vertebrates.

The hypophysis (pituitary body) (Fig. 721) is a reddish-gray, somewhat oval mass, measuring about 12.5 mm. in its transverse, and about 8 mm. in its antero-posterior diameter. It is attached to the end of the infundibulum, and is situated in the fossa hypophyseos of the sphenoidal bone, where it is retained by a circular fold of dura mater, the diaphragma sella; this fold almost completely roofs in the fossa, leaving only a small central aperture through which the infundibulum passes.

Optic Chiasma (chiasma opticum; optic commissure).The optic chiasma is a flattened, somewhat quadrilateral band of fibers, situated at the junction of the floor and anterior wall of the third ventricle. Most of its fibers have their origins in the retina, and reach the chiasma through the optic nerves, which are continuous with its antero-lateral angles. In the chiasma, they undergo a partial decussation (Fig. 722); the fibers from the nasal half of the retina decussate and enter the optic tract of the opposite side, while the fibers from the temporal half of the retina do not undergo decussation, but pass back into the optic tract of the same side. Occupying the posterior part of the commissure, however, is a strand of fibers, the commissure of Gudden, which is not derived from the optic nerves; it forms a connecting link between the medial geniculate bodies.

Optic Tracts.The optic tracts are continued backward and lateralward from the postero-lateral angles of the optic chiasma. Each passes between the anterior perforated substance and the tuber cinereum, and, winding around the ventrolateral aspect of the cerebral peduncle, divides into a medial and a lateral root. The former comprises the fibers of Guddens commissure. The lateral root consists mainly of afferent fibers which arise in the retina and undergo partial decussation in the optic chiasma, as described; but it also contains a few fine efferent fibers which have their origins in the brain and their terminations in the retina. When traced backward, the afferent fibers of the lateral root are found to end in the lateral geniculate body and pulvinar of the thalamus, and in the superior colliculus; and these three structures constitute the lower visual centers. Fibers arise from the nerve cells in these centers and pass through the occipital part of the internal capsule, under the name of the optic radiations, to the cortex of the occipital lobe of the cerebrum, where the higher or cortical visual center is situated. Some of the fibers of the optic radiations take an opposite course, arising from the cells of the occipital cortex and passing to the lower visual centers. Some fibers are detached from the optic tract, and pass through the cerebral peduncle to the nucleus of the oculomotor nerve. These may be regarded as the afferent branches for the Sphincter pupillæ and Ciliaris muscles. Other fibers have been described as reaching the cerebellum through the superior peduncle; while others, again, are lost in the pons.

The Third Ventricle (ventriculus tertius) (Figs. 716,720).The third ventricle is a median cleft between the two thalami. Behind, it communicates with the fourth ventricle through the cerebral aqueduct, and in front with the lateral ventricles through the interventricular foramen. Somewhat triangular in shape, with the apex directed backward, it has a roof, a floor, an anterior and a posterior boundary and a pair of lateral walls.

The roof(Fig. 723) is formed by a layer of epithelium, which stretches between the upper edges of the lateral walls of the cavity and is continuous with the epithelial lining of the ventricle. It is covered by and adherent to a fold of pia mater, named the tela chorioidea of the third ventricle, from the under surface of which a pair of vascular fringed processes, the choroid plexuses of the third ventricle, project downward, one on either side of the middle line, and invaginate the epithelial roof into the ventricular cavity.

The floor slopes downward and forward and is formed mainly by the structures which constitute the hypothalamus: from before backward these are: the optic chiasma, the tuber cinereum and infundibulum, and the corpora mammillaria. Behind the last, the floor is formed by the interpeduncular fossa and the tegmenta of the cerebral peduncles. The ventricle is prolonged downward as a funnel-shaped recess, the recessus infundibuli, into the infundibulum, and to the apex of the latter the hypophysis is attached.

The anterior boundary is constituted below by the lamina terminalis, a thin layer of gray substance stretching from the upper surface of the optic chiasma to the rostrum of the corpus callosum; above by the columns of the fornix and the anterior commissure. At the junction of the floor and anterior wall, immediately above the optic chiasma, the ventricle presents a small angular recess or diverticulum, the optic recess. Between the columns of the fornix, and above the anterior commissure, is a second recess termed the vulva. At the junction of the roof and anterior wall of the ventricle, and situated between the thalami behind and the columns of the fornix in front, is the interventricular foramen (foramen of Monro) through which the third communicates with the lateral ventricles.

The posterior boundary is constituted by the pineal body, the posterior commissure and the cerebral aqueduct. A small recess, the recessus pinealis, projects into the stalk of the pineal body, while in front of and above the pineal body is a second recess, the recessus suprapinealis, consisting of a diverticulum of the epithelium which forms the ventricular roof.

Each lateral wall consists of an upper portion formed by the medial surface of the anterior two-thirds of the thalamus, and a lower consisting of an upward continuation of the gray substance of the ventricular floor. These two parts correspond to the alar and basal laminæ respectively of the lateral wall of the fore-brain vesicle and are separated from each other by a furrow, the sulcus of Monro, which extends from the interventricular foramen to the cerebral aqueduct (pages 741 and 742). The lateral wall is limited above by the tænia thalami. The columns of the fornix curve downward in front of the interventricular foramen, and then run in the lateral walls of the ventricle, where, at first, they form distinct prominences, but subsequently are lost to sight. The lateral walls are joined to each other across the cavity of the ventricle by a band of gray matter, the massa intermedia (page 809).

Interpeduncular Fossa (Fig. 724).This is a somewhat lozenge-shaped area of the base of the brain, limited in front by the optic chiasma, behind by the antero-superior surface of the pons, antero-laterally by the converging optic tracts, and postero-laterally by the diverging cerebral peduncles. The structures contained in it have already been described; from behind forward, they are the posterior perforated substance, corpora mamillaria, tuber cinereum, infundibulum, and hypophysis.

The Telencephalon.The telencephalon includes: (1) the cerebral hemispheres with their cavities, the lateral ventricles; and (2) the pars optica hypothalami and the anterior portion of the third ventricle (already described under the diencephalon). As previously stated (see page 744), each cerebral hemisphere may be divided into three fundamental parts, viz., the rhinencephalon, the corpus striatum, and the neopallium. The rhinencephalon, associated with the sense of smell, is the oldest part of the telencephalon, and forms almost the whole of the hemisphere in some of the lower animals, e. g., fishes, amphibians, and reptiles. In man it is rudimentary, whereas the neopallium undergoes great development and forms the chief part of the hemisphere.

The Cerebral Hemispheres.The cerebral hemispheres constitute the largest part of the brain, and, when viewed together from above, assume the form of an ovoid mass broader behind than in front, the greatest transverse diameter corresponding with a line connecting the two parietal eminences. The hemispheres are separated medially by a deep cleft, named the longitudinal cerebral fissure, and each possesses a central cavity, the lateral ventricle.

The Longitudinal Cerebral Fissure (fissura cerebri longitudinalis; great longitudinal fissure) contains a sickle-shaped process of dura mater, the falx cerebri. It front and behind, the fissure extends from the upper to the under surfaces of the hemispheres and completely separates them, but its middle portion separates them for only about one-half of their vertical extent; for at this part they are connected across the middle line by a great central white commissure, the corpus callosum.

In a median sagittal section (Fig. 720) the cut corpus callosum presents the appearance of a broad, arched band. Its thick posterior end, termed the splenium, overlaps the mid-brain, but is separated from it by the tela chorioidea of the third ventricle and the pineal body. Its anterior curved end, termed the genu, gradually tapers into a thinner portion, the rostrum, which is continued downward and backward in front of the anterior commissure to join the lamina terminalis. Arching backward from immediately behind the anterior commissure to the under surface of the splenium is a second white band named the fornix: between this and the corpus callosum are the laminæ and cavity of the septum pellucidum.

The lateral surface is convex in adaptation to the concavity of the corresponding half of the vault of the cranium. The medial surface is flat and vertical, and is separated from that of the opposite hemisphere by the great longitudinal fissure and the falx cerebri. The inferior surface is of an irregular form, and may be divided into three areas: anterior, middle, and posterior. The anterior area, formed by the orbital surface of the frontal lobe, is concave, and rests on the roof of the orbit and nose; the middle area is convex, and consists of the under surface of the temporal lobe: it is adapted to the corresponding half of the middle cranial fossa. The posterior area is concave, directed medialward as well as downward, and is named the tentorial surface, since it rests upon the tentorium cerebelli, which intervenes between it and the upper surface of the cerebellum.

These three surfaces are separated from each other by the following borders: (a) supero-medial, between the lateral and medial surfaces; (b) infero-lateral, between the lateral and inferior surfaces; the anterior part of this border separating the lateral from the orbital surface, is known as the superciliary border; (c) medial occipital, separating the medial and tentorial surfaces; and (d) medial orbital, separating the orbital from the medial surface. The anterior end of the hemisphere is named the frontal pole; the posterior, the occipital pole; and the anterior end of the temporal lobe, the temporal pole. About 5 cm. in front of the occipital pole on the infero-lateral border is an indentation or notch, named the preoccipital notch.

The surfaces of the hemispheres are moulded into a number of irregular eminences, named gyri or convolutions, and separated by furrows termed fissures and sulci. The furrows are of two kinds, complete and incomplete. The former appear early in fetal life, are few in number, and are produced by infoldings of the entire thickness of the brain wall, and give rise to corresponding elevations in the interior of the ventricle. They comprise the hippocampal fissure, and parts of the calcarine and collateral fissures. The incomplete furrows are very numerous, and only indent the subjacent white substance, without producing any corresponding elevations in the ventricular cavity.

The gyri and their intervening fissures and the sulci are fairly constant in their arrangement; at the same time they vary within certain limits, not only in different individuals, but on the two hemispheres of the same brain. The convoluted condition of the surface permits of a great increase of the gray matter without the sacrifice of much additional space. The number and extent of the gyri, as well as the depth of the intervening furrows, appear to bear a direct relation to the intellectual powers of the individual.

Certain of the fissures and sulci are utilized for the purpose of dividing the hemisphere into lobes, and are therefore termed interlobular; included under this category are the lateral cerebral, parietoöccipital, calcarine, and collateral fissures, the central and cingulate sulci, and the sulcus circularis.

The Lateral Cerebral Fissure (fissura cerebri lateralis [Sylvii]; fissure of Sylvius) (Fig. 726) is a well-marked cleft on the inferior and lateral surfaces of the hemisphere, and consists of a short stem which divides into three rami. The stem is situated on the base of the brain, and commences in a depression at the lateral angle of the anterior perforated substance. From this point it extends between the anterior part of the temporal lobe and the orbital surface of the frontal lobe, and reaches the lateral surface of the hemisphere. Here it divides into three rami: an anterior horizontal, an anterior ascending, and a posterior. The anterior horizontal ramus passes foward for about 2.5 cm. into the inferior frontal gyrus, while the anterior ascending ramus extends upward into the same convolution for about an equal distance. The posterior ramus is the longest; it runs backward and slightly upward for about 7 cm., and ends by an upward inflexion in the parietal lobe.

The Central Sulcus (sulcus centralis [Rolandi]; fissure of Rolando; central fissure) (Figs. 725,726) is situated about the middle of the lateral surface of the hemisphere, and begins in or near the longitudinal cerebral fissure, a little behind its mid-point. It runs sinuously downward and forward, and ends a little above the posterior ramus of the lateral fissure, and about 2.5 cm. behind the anterior ascending ramus of the same fissure. It described two chief curves: a superior genu with its concavity directed forward, and an inferior genu with its concavity directed backward. The central sulcus forms an angle opening forward of about 70° with the median plane.

The medial part of the parietoöccipital fissure (Fig. 727) runs downward and forward as a deep cleft on the medial surface of the hemisphere, and joins the calcarine fissure below and behind the posterior end of the corpus callosum. In most cases it contains a submerged gyrus.

The Calcarine Fissure (fissura calcarina) (Fig. 727) is on the medial surface of the hemisphere. It begins near the occipital pole in two converging rami, and runs forward to a point a little below the splenium of the corpus callosum, where it is joined at an acute angle by the medial part of the parietoöccipital fissure. The anterior part of this fissure gives rise to the prominence of the calcar avis in the posterior cornu of the lateral ventricle.

The Cingulate Sulcus (sulcus cinguli; callosomarginal fissure) (Fig. 727) is on the medial surface of the hemisphere; it begins below the anterior end of the corpus callosum and runs upward and forward nearly parallel to the rostrum of this body and, curving in front of the genu, is continued backward above the corpus callosum, and finally ascends to the supero-medial border of the hemisphere a short distance behind the upper end of the central sulcus. It separates the superior frontal from the cingulate gyrus.

The Collateral Fissure (fissura collateralis) (Fig. 727) is on the tentorial surface of the hemisphere and extends from near the occipital pole to within a short distance of the temporal pole. Behind, it lies below and lateral to the calcarine fissure, from which it is separated by the lingual gyrus; in front, it is situated between the hippocampal gyrus and the anterior part of the fusiform gyrus.

The Sulcus Circularis (circuminsular fissure) (Fig. 731) is on the lower and lateral surfaces of the hemisphere: it surrounds the insula and separates it from the frontal, parietal, and temporal lobes.

Lobes of the Hemispheres.By means of these fissures and sulci, assisted by certain arbitrary lines, each hemisphere is divided into the following lobes: the frontal, the parietal, the temporal, the occipital, the limbic, and the insula.

Frontal Lobe (lobus frontalis).On the lateral surface of the hemisphere this lobe extends from the frontal pole to the central sulcus, the latter separating it from the parietal lobe. Below, it is limited by the posterior ramus of the lateral fissure, which intervenes between it and the central lobe. On the medial surface, it is separated from the cingulate gyrus by the cingulate sulcus; and on the inferior surface, it is bounded behind by the stem of the lateral fissure.

The lateral surface of the frontal lobe (Fig. 726) is tranversed by three sulci which divide it into four gyri: the sulci are named the precentral, and the superior and inferior frontal; the gyri are the anterior central, and the superior, middle, and inferior frontal. The precentral sulcus runs parallel to the central sulcus, and is usually divided into an upper and a lower part; between it and the central sulcus is the anterior central gyrus. From the precentral sulcus, the superior and inferior frontal sulci run forward and downward, and divide the remainder of the lateral surface of the lobe into three parallel gyri, named, respectively the superior, middle, and inferior frontal gyri.

The anterior central gyrus (gyrus centralis anterior; ascending frontal convolution; precentral gyre) is bounded in front by the precentral sulcus, behind by the central sulcus; it extends from the supero-medial border of the hemisphere to the posterior ramus of the lateral fissure.

The superior frontal gyrus (gyrus frontalis superior; superfrontal gyre) is situated above the superior frontal sulcus and is continued on to the medial surface of the hemisphere. The portion on the lateral surface of the hemisphere is usually more or less completely subdivided into an upper and a lower part by an antero-posterior sulcus, the paramedial sulcus, which, however, is frequently interrupted by bridging gyri.

The middle frontal gyrus (gyrus frontalis medius; medifrontal gyre), between the superior and inferior frontal sulci, is continuous with the anterior orbital gyrus on the inferior surface of the hemisphere; it is frequently subdivided into two by a horizontal sulcus, the medial frontal sulcus of Eberstaller, which ends anteriorly in a wide bifurcation.

The inferior frontal gyrus (gyrus frontalis inferior; subfrontal gyre) lies below the inferior frontal sulcus, and extends forward from the lower part of the precentral sulcus; it is continuous with the lateral and posterior orbital gyri on the under surface of the lobe. It is subdivided by the anterior horizontal and ascending rami of the lateral fissure into three parts, viz., (1) the orbital part, below the anterior horizontal ramus of the fissure; (2) the triangular part (cap of Broca), between the ascending and horizontal rami; and (3) the basilar part, behind the anterior ascending ramus. The left inferior frontal gyrus is, as a rule, more highly developed than the right, and is named the gyrus of Broca, from the fact that Broca described it as the center for articulate speech.

The inferior or orbital surface of the frontal lobe is concave, and rests on the orbital plate of the frontal bone (Fig. 729). It is divided into four orbital gyri by a well-marked H-shaped orbital sulcus. These are named, from their position, the medial, anterior, lateral, and posterior orbital gyri. The medial orbital gyrus presents a well-marked antero-posterior sulcus, the olfactory sulcus, for the olfactory tract; the portion medial to this is named the straight gyrus, and is continuous with the superior frontal gyrus on the medial surface.

The medial surface of the frontal lobe is occupied by the medial part of the superior frontal gyrus (marginal gyrus) (Fig. 727). It lies between the cingulate sulcus and the supero-medial margin of the hemisphere. The posterior part of this gyrus is sometimes marked off by a vertical sulcus, and is distinguished as the paracentral lobule, because it is continuous with the anterior and posterior central gyri.

Parietal Lobe (lobus parietalis).The parietal lobe is separated from the frontal lobe by the central sulcus, but its boundaries below and behind are not so definite. Posteriorly, it is limited by the parietoöccipital fissure, and by a line carried across the hemisphere from the end of this fissure toward the preoccipital notch. Below, it is separated from the temporal lobe by the posterior ramus of the lateral fissure, and by a line carried backward from it to meet the line passing downward to the preoccipital notch.

The lateral surface of the parietal lobe (Fig. 726) is cleft by a well-marked furrow, the intraparietal sulcus of Turner, which consists of an oblique and a horizontal portion. The oblique part is named the postcentral sulcus, and commences below, about midway between the lower end of the central sulcus and the upturned end of the lateral fissure. It runs upward and backward, parallel to the central sulcus, and is sometimes divided into an upper and a lower ramus. It forms the hinder limit of the posterior central gyrus.

From about the middle of the postcentral sulcus, or from the upper end of its inferior ramus, the horizontal portion of the intraparietal sulcus is carried backward and slightly upward on the parietal lobe, and is prolonged, under the name of the occipital ramus, on to the occipital lobe, where it divides into two parts, which form nearly a right angle with the main stem and constitute the transverse occipital sulcus. The part of the parietal lobe above the horizontal portion of the intraparietal sulcus is named the superior parietal lobule; the part below, the inferior parietal lobule.

The posterior central gyrus (gyrus centralis posterior; ascending parietal convolution; postcentral gyre) extends from the longitudinal fissure above to the posterior ramus of the lateral fissure below. It lies parallel with the anterior central gyrus, with which it is connected below, and also, sometimes, above, the central sulcus.

The superior parietal lobule (lobulus parietalis superior) is bounded in front by the upper part of the postcentral sulcus, but is usually connected with the posterior central gyrus above the end of the sulcus; behind it is the lateral part of the parietoöccipital fissure, around the end of which it is joined to the occipital lobe by a curved gyrus, the arcus parietoöccipitalis; below, it is separated from the inferior parietal lobule by the horizontal portion of the intraparietal sulcus.

The inferior parietal lobule (lobulus parietalis inferior; subparietal district or lobule) lies below the horizontal portion of the intraparietal sulcus, and behind the lower part of the postcentral sulcus. It is divided from before backward into two gyri. One, the supramarginal, arches over the upturned end of the lateral fissure; it is continuous in front with the postcentral gyrus, and behind with the superior temporal gyrus. The second, the angular, arches over the posterior end of the superior temporal sulcus, behind which it is continuous with the middle temporal gyrus.

The medial surface of the parietal lobe (Fig. 727) is bounded behind by the medial part of the parietoöccipital fissure; in front, by the posterior end of the cingulate sulcus; and below, it is separated from the cingulate gyrus by the subparietal sulcus. It is of small size, and consists of a square-shaped convolution, which is termed the precuneus or quadrate lobe.

The lateral surface is limited in front by the lateral part of the parietoöccipital fissure, and by a line carried from the end of this fissure to the preoccipital notch; it is traversed by the transverse occipital and the lateral occipital sulci. The transverse occipital sulcus is continuous with the posterior end of the occipital ramus of the intraparietal sulcus, and runs across the upper part of the lobe, a short distance behind the parietoöccipital fissure. The lateral occipital sulcus extends from behind forward, and divides the lateral surface of the occipital lobe into a superior and an inferior gyrus, which are continuous in front with the parietal and temporal lobes.125

The medial surface of the occipital lobe is bounded in front by the medial part of the parietoöccipital fissure, and is traversed by the calcarine fissure, which subdivides it into the cuneus and the lingual gyrus. The cuneus is a wedge-shaped area between the calcarine fissure and the medial part of the parietoöccipital fissure. The lingual gyrus lies between the calcarine fissure and the posterior part of the collateral fissure; behind, it reaches the occipital pole; in front, it is continued on to the tentorial surface of the temporal lobe, and joins the hippocampal gyrus.

The tentorial surface of the occipital lobe is limited in front by an imaginary transverse line through the preoccipital notch, and consists of the posterior part of the fusiform gyrus (occipitotemporal convolution) and the lower part of the lingual gyrus, which are separated from each other by the posterior segment of the collateral fissure.

The superior surface forms the lower limit of the lateral fissure and overlaps the insula. On opening out the lateral fissure, three or four gyri will be seen springing from the depth of the hinder end of the fissure, and running obliquely forward and outward on the posterior part of the upper surface of the superior temporal gyrus; these are named the transverse temporal gyri (Heschl) (Fig. 730).

The lateral surface(Fig. 726) is bounded above by the posterior ramus of the lateral fissure, and by the imaginary line continued backward from it; below, it is limited by the infero-lateral border of the hemisphere. It is divided into superior, middle, and inferior gyri by the superior and middle temporal sulci. The superior temporal sulcus runs from before backward across the temporal lobe, some little distance below, but parallel with, the posterior ramus of the lateral fissure; and hence it is often termed the parallel sulcus. The middle temporal sulcus takes the same direction as the superior, but is situated at a lower level, and is usually subdivided into two or more parts. The superior temporal gyrus lies between the posterior ramus of the lateral fissure and the superior temporal sulcus, and is continuous behind with the supramarginal and angular gyri. The middle temporal gyrus is placed between the superior and middle temporal sulci, and is joined posteriorly with the angular gyrus. The inferior temporal gyrus is placed below the middle temporal sulcus, and is connected behind with the inferior occipital gyrus; it also extends around the infero-lateral border on to the inferior surface of the temporal lobe, where it is limited by the inferior sulcus.

The inferior surface is concave, and is continuous posteriorly with the tentorial surface of the occipital lobe. It is traversed by the inferior temporal sulcus, which extends from near the occipital pole behind, to within a short distance of the temporal pole in front, but is frequently subdivided by bridging gyri. Lateral to this fissure is the narrow tentorial part of the inferior temporal gyrus, and medial to it the fusiform gyrus, which extends from the occipital to the temporal pole; this gyrus is limited medially by the collateral fissure, which separates it from the lingual gyrus behind and from the hippocampal gyrus in front.

The Insula (island of Reil; central lobe) (Fig. 731) lies deeply in the lateral or Sylvian fissure, and can only be seen when the lips of that fissure are widely separated, since it is overlapped and hidden by the gyri which bound the fissure. These gyri are termed the opercula of the insula; they are separated from each other by the three rami of the lateral fissure, and are named the orbital, frontal, frontoparietal, and temporal opercula. The orbital operculum lies below the anterior horizontal ramus of the fissure, the frontal between this and the anterior ascending ramus, the parietal between the anterior ascending ramus and the upturned end of the posterior ramus, and the temporal below the posterior ramus. The frontal operculum is of small size in those cases where the anterior horizontal and ascending rami of the lateral fissure arise from a common stem. The insula is surrounded by a deep circular sulcus which separates it from the frontal, parietal, and temporal lobes. When the opercula have been removed, the insula is seen as a triangular eminence, the apex of which is directed toward the anterior perforated substance. It is divided into a larger anterior and a smaller posterior part by a deep sulcus, which runs backward and upward from the apex of the insula. The anterior part is subdivided by shallow sulci into three or four short gyri, while the posterior part is formed by one long gyrus, which is often bifurcated at its upper end. The cortical gray substance of the insula is continuous with that of the different opercula, while its deep surface corresponds with the lentiform nucleus of the corpus striatum.

FIG. 731 The insula of the left side, exposed by removing the opercula. (See enlarged image)

Limbic Lobe (Fig. 727).The term limbic lobe was introduced by Broca, and under it he included the cingulate and hippocampal gyri, which together arch around the corpus callosum and the hippocampal fissure. These he separated on the morphological ground that they are well-developed in animals possessing a keen sense of smell (osmatic animals), such as the dog and fox. They were thus regarded as a part of the rhinencephalon, but it is now recognized that they belong to the neopallium; the cingulate gyrus is therefore sometimes described as a part of the frontal lobe, and the hippocampal as a part of the temporal lobe.

The cingulate gyrus (gyrus cinguli; callosal convolution) is an arch-shaped convolution, lying in close relation to the superficial surface of the corpus callosum, from which it is separated by a slit-like fissure, the callosal fissure. It commences below the rostrum of the corpus callosum, curves around in front of the genu, extends along the upper surface of the body, and finally turns downward behind the splenium, where it is connected by a narrow isthmus with the hippocampal gyrus. It is separated from the medial part of the superior frontal gyrus by the cingulate sulcus, and from the precuneus by the subparietal sulcus.

The hippocampal gyrus (gyrus hippocampi) is bounded above by the hippocampal fissure, and below by the anterior part of the collateral fissure. Behind, it is continuous superiorly, through the isthmus, with the cingulate gyrus and inferiorly with the lingual gyrus. Running in the substance of the cingulate and hippocampal gyri, and connecting them together, is a tract of arched fibers, named the cingulum (page 843). The anterior extremity of the hippocampal gyrus is recurved in the form of a hook (uncus), which is separated from the apex of the temporal lobe by a slight fissure, the incisura temporalis. Although superficially continuous with the hippocampal gyrus, the uncus forms morphologically a part of the rhinencephalon.

The Hippocampal Fissure (fissura hippocampi; dentate fissure) begins immediately behind the splenium of the corpus callosum, and runs forward between the hippocampal and dentate gyri to end in the uncus. It is a complete fissure (page 819), and gives rise to the prominence of the hippocampus in the inferior cornu of the lateral ventricle.

Rhinencephalon (Fig. 732).The rhinencephalon comprises the olfactory lobe, the uncus, the subcallosal and supracallosal gyri, the fascia dentata hippocampi, the septum pellucidum, the fornix, and the hippocampus.

1. The Olfactory Lobe (lobus olfactorius) is situated under the inferior or orbital surface of the frontal lobe. In many vertebrates it constitutes a well-marked portion of the hemisphere and contains an extension of the lateral ventricle; but in man and some other mammals it is rudimentary. It consists of the olfactory bulb and tract, the olfactory trigone, the parolfactory area of Broca, and the anterior perforated substance.

(a) The olfactory bulb (bulbus olfactorius) is an oval, reddish-gray mass which rests on the cribriform plate of the ethmoid and forms the anterior expanded extremity of the olfactory tract. Its under surface receives the olfactory nerves, which pass upward through the cribriform plate from the olfactory region of the nasal cavity. Its minute structure is described on page 848.

(b) The olfactory tract (tractus olfactorius) is a narrow white band, triangular on coronal section, the apex being directed upward. It lies in the olfactory sulcus on the inferior surface of the frontal lobe, and divides posteriorly into two striæ, a medial and a lateral. The lateral stria is directed across the lateral part of the anterior perforated substance and then bends abruptly medialward toward the uncus of the hippocampal gyrus. The medial stria turns medialward behind the parolfactory area and ends in the subcallosal gyrus; in some cases a small intermediate stria is seen running backward to the anterior perforated substance.

(c) The olfactory trigone (trigonum olfactorium) is a small triangular area in front of the anterior perforated substance. Its apex, directed forward, occupies the posterior part of the olfactory sulcus, and is brought into view by throwing back the olfactory tract.

(d) The parolfactory area of Broca (area parolfactoria) is a small triangular field on the medial surface of the hemisphere in front of the subcallosal gyrus, from which it is separated by the posterior parolfactory sulcus; it is continuous below with the olfactory trigone, and above and in front with the cingulate gyrus; it is limited anteriorly by the anterior parolfactory sulcus.

(e) The anterior perforated substance (substantia perforata anterior) is an irregularly quadrilateral area in front of the optic tract and behind the olfactory trigone, from which it is separated by the fissure prima; medially and in front it is continuous with the subcallosal gyrus; laterally it is bounded by the lateral stria of the olfactory tract and is continued into the uncus. Its gray substance is confluent above with that of the corpus striatum, and is perforated anteriorly by numerous small bloodvessels.

3. The Subcallosal, Supracallosal, and Dentate Gyri form a rudimentary arch-shaped lamina of gray substance extending over the corpus callosum and above the hippocampal gyrus from the anterior perforated substance to the uncus.

(a) The subcallosal gyrus (gyrus subcallosus; peduncle of the corpus callosum) is a narrow lamina on the medial surface of the hemisphere in front of the lamina terminalis, behind the parolfactory area, and below the rostrum of the corpus callosum. It is continuous around the genu of the corpus callosum with the supracallosal gyrus.

(b) The supracallosal gyrus (indusium griseum; gyrus epicallosus) consists of a thin layer of gray substance in contact with the upper surface of the corpus callosum and continuous laterally with the gray substance of the cingulate gyrus. It contains two longitudinally directed strands of fibers termed respectively the medial and lateral longitudinal striæ. The supracallosal gyrus is prolonged around the splenium of the corpus callosum as a delicate lamina, the fasciola cinerea, which is continuous below with the fascia dentata hippocampi.

(c) The fascia dentata hippocampi (gyrus dentatus) is a narrow band extending downward and forward above the hippocampal gyrus but separated from it by the hippocampal fissure; its free margin is notched and overlapped by the fimbriathe fimbriodentate fissure intervening. Anteriorly it is continued into the notch of the uncus, where it forms a sharp bend and is then prolonged as a delicate band, the band of Giacomini, over the uncus, on the lateral surface of which it is lost.

Interior of the Cerebral Hemispheres.If the upper part of either hemisphere be removed, at a level about 1.25 cm. above the corpus callosum, the central white substance will be exposed as an oval-shaped area, the centrum ovale minus, surrounded by a narrow convoluted margin of gray substance, and studded with numerous minute red dots (puncta vasculosa), produced by the escape of blood from divided bloodvessels. If the remaining portions of the hemispheres be slightly drawn apart a broad band of white substance, the corpus callosum, will be observed, connecting them at the bottom of the longitudinal fissure; the margins of the hemispheres which overlap the corpus callosum are called the labia cerebri. Each labrium is part of the cingulate gyrus already described; and the slit-like interval between it and the upper surface of the corpus callosum is termed the callosal fissure(Fig. 727). If the hemispheres be sliced off to a level with the upper surface of the corpus callosum, the white substance of that structure will be seen connecting the two hemispheres. The large expanse of medullary matter now exposed, surrounded by the convoluted margin of gray substance, is called the centrum ovale majus.

The Corpus Callosum(Fig. 733) is the great transverse commissure which unites the cerebral hemispheres and roofs in the lateral ventricles. A good conception of its position and size is obtained by examining a median sagittal section of the brain (Fig. 720), when it is seen to form an arched structure about 10 cm. long. Its anterior end is about 4 cm. from the frontal pole, and its posterior end about 6 cm. from the occipital pole of the hemisphere.

The anterior end is named the genu, and is bent downward and backward in front of the septum pellucidum; diminishing rapidly in thickness, it is prolonged backward under the name of the rostrum, which is connected below with the lamina terminalis. The anterior cerebral arteries are in contact with the under surface of the rostrum; they then arch over the front of the genu, and are carried backward above the body of the corpus callosum.

The posterior end is termed the splenium and constitutes the thickest part of the corpus callosum. It overlaps the tela chorioidea of the third ventricle and the mid-brain, and ends in a thick, convex, free border. A sagittal section of the splenium shows that the posterior end of the corpus callosum is acutely bent forward, the upper and lower parts being applied to each other.

The superior surface is convex from before backward, and is about 2.5 cm. wide. Its medial part forms the bottom of the longitudinal fissure, and is in contact posteriorly with the lower border of the falx cerebri. Laterally it is overlapped by the cingulate gyrus, but is separated from it by the slit-like callosal fissure. It is traversed by numerous transverse ridges and furrows, and is covered by a thin layer of gray matter, the supracallosal gyrus, which exhibits on either side of the middle line the medial and lateral longitudinal striæ, already described (page 827).

The inferior surface is concave, and forms on either side of the middle line the roof of the lateral ventricle. Medially, this surface is attached in front to the septum pellucidum; behind this it is fused with the upper surface of the body of the fornix, while the splenium is in contact with the tela chorioidea.

On either side, the fibers of the corpus callosum radiate in the white substance and pass to the various parts of the cerebral cortex; those curving forward from the genu into the frontal lobe constitute the forceps anterior, and those curving backward into the occipital lobe, the forceps posterior. Between these two parts is the main body of the fibers which constitute the tapetum and extend laterally on either side into the temporal lobe, and cover in the central part of the lateral ventricle.

FIG. 734 Scheme showing relations of the ventricles to the surface of the brain. (See enlarged image)

The Lateral Ventricles (ventriculus lateralis) (Fig. 734).The two lateral ventricles are irregular cavities situated in the lower and medial parts of the cerebral hemispheres, one on either side of the middle line. They are separated from each other by a median vertical partition, the septum pellucidum, but communicate with the third ventricle and indirectly with each other through the interventricular foramen. They are lined by a thin, diaphanous membrane, the ependyma, covered by ciliated epithelium, and contain cerebrospinal fluid, which, even in health, may be secreted in considerable amount. Each lateral ventricle consists of a central part or body, and three prolongations from it, termed cornua (Figs. 735,736).

The central part (pars centralis ventriculi lateralis; cella) (Fig. 737) of the lateral ventricle extends from the interventricular foramen to the splenium of the corpus callosum. It is an irregularly curved cavity, triangular on transverse section, with a roof, a floor, and a medial wall. The roof is formed by the under surface of the corpus callosum; the floor by the following parts, enumerated in their order of position, from before backward: the caudate nucleus of the corpus striatum, the stria terminalis and the terminal vein, the lateral portion of the upper surface of the thalamus, the choroid plexus, and the lateral part of the fornix; the medial wall is the posterior part of the septum pellucidum, which separates it from the opposite ventricle.

FIG. 736 Drawing of a cast of the ventricular cavities, viewed from the side. (Retzius.) (See enlarged image)

The anterior cornu (cornu anterius; anterior horn; precornu) (Fig. 736) passes forward and lateralward, with a slight inclination downward, from the interventricular foramen into the frontal lobe, curving around the anterior end of the caudate nucleus. Its floor is formed by the upper surface of the reflected portion of the corpus callosum, the rostrum. It is bounded medially by the anterior portion of the septum pellucidum, and laterally by the head of the caudate nucleus. Its apex reaches the posterior surface of the genu of the corpus callosum.

The posterior cornu (cornu posterius; postcornu) (Figs. 737,788) passes into the occipital lobe, its direction being backward and lateralward, and then medialward. Its roof is formed by the fibers of the corpus callosum passing to the temporal and occipital lobes. On its medial wall is a longitudinal eminence, the calcar avis (hippocampus minor), which is an involution of the ventricular wall produced by the calcarine fissure. Above this the forceps posterior of the corpus callosum, sweeping around to enter the occipital lobe, causes another projection, termed the bulb of the posterior cornu. The calcar avis and bulb of the posterior cornu are extremely variable in their degree of development; in some cases they are ill-defined, in others prominent.

The inferior cornu (cornu inferior; descending horn; middle horn; medicornu) (Fig. 739), the largest of the three, traverses the temporal lobe of the brain, forming in its course a curve around the posterior end of the thalamus. It passes at first backward, lateralward, and downward, and then curves forward to within 2.5 cm. of the apex of the temporal lobe, its direction being fairly well indicated on the surface of the brain by that of the superior temporal sulcus. Its roof is formed chiefly by the inferior surface of the tapetum of the corpus callosum, but the tail of the caudate nucleus and the stria terminalis also extend forward in the roof of the inferior cornu to its extremity; the tail of the caudate nucleus joins the putamen. Its floor presents the following parts: the hippocampus, the fimbria hippocampi, the collateral eminence, and the choroid plexus. When the choroid plexus is removed, a cleft-like opening is left along the medial wall of the inferior cornu; this cleft constitutes the lower part of the choroidal fissure.

The hippocampus (hippocampus major) (Figs. 739,740) is a curved eminence, about 5 cm. long, which extends throughout the entire length of the floor of the inferior cornu. Its lower end is enlarged, and presents two or three rounded elevations or digitations which give it a paw-like appearance, and hence it is named the pes hippocampi. If a transverse section be made through the hippocampus, it will be seen that this eminence is produced by the folding of the wall of the hemisphere to form the hippocampal fissure. The main mass of the hippocampus consists of gray substance, but on its ventricular surface is a thin white layer, the alveus, which is continuous with the fimbria hippocampi.

The collateral eminence (eminentia collateralis) (Fig. 740) is an elongated swelling lying lateral to and parallel with the hippocampus. It corresponds with the middle part of the collateral fissure, and its size depends on the depth and direction of this fissure. It is continuous behind with a flattened triangular area, the trigonum collaterale, situated between the posterior and inferior cornua.

The corpus striatum has received its name from the striped appearance which a section of its anterior part presents, in consequence of diverging white fibers being mixed with the gray substance which forms its chief mass. A part of the corpus striatum is imbedded in the white substance of the hemisphere, and is therefore external to the ventricle; it is termed the extraventricular portion, or the lentiform nucleus; the remainder, however, projects into the ventricle, and is named the intraventricular portion, or the caudate nucleus(Fig. 737).

The caudate nucleus (nucleus caudatus; caudatum) (Figs. 741,742) is a pear-shaped, highly arched gray mass; its broad extremity, or head, is directed forward into the anterior cornu of the lateral ventricle, and is continuous with the anterior perforated substance and with the anterior end of the lentiform nucleus; its narrow end, or tail, is directed backward on the lateral side of the thalamus, from which it is separated by the stria terminalis and the terminal vein. It is then continued downward into the roof of the inferior cornu, and ends in the putamen near the apex of the temporal lobe. It is covered by the lining of the ventricle, and crossed by some veins of considerable size. It is separated from the lentiform nucleus, in the greater part of its extent, by a thick lamina of white substance, called the internal capsule, but the two portions of the corpus striatum are united in front (Figs. 743,744).

The lentiform nucleus (nucleus lentiformis; lenticular nucleus; lenticula) (Fig. 741) is lateral to the caudate nucleus and thalamus, and is seen only in sections of the hemisphere. When divided horizontally, it exhibits, to some extent, the appearance of a biconvex lens (Fig. 742), while a coronal section of its central part presents a somewhat triangular outline. It is shorter than the caudate nucleus and does not extend as far forward. It is bounded laterally by a lamina of white substance called the external capsule, and lateral to this is a thin layer of gray substance termed the claustrum. Its anterior end is continuous with the lower part of the head of the caudate nucleus and with the anterior perforated substance.

In a coronal section through the middle of the lentiform nucleus, two medullary laminæ are seen dividing it into three parts. The lateral and largest part is of a reddish color, and is known as the putamen, while the medial and intermediate are of a yellowish tint, and together constitute the globus pallidus; all three are marked by fine radiating white fibers, which are most distinct in the putamen (Fig. 744).

The gray substance of the corpus striatum is traversed by nerve fibers, some of which originate in it. The cells are multipolar, both large and small; those of the lentiform nucleus contain yellow pigment. The caudate and lentiform nuclei are not only directly continuous with each other anteriorly, but are connected to each other by numerous fibers. The corpus striatum is also connected: (1) to the cerebral cortex, by what are termed the corticostriate fibers; (2) to the thalamus, by fibers which pass through the internal capsule, and by a strand named the ansa lentiformis; (3) to the cerebral peduncle, by fibers which leave the lower aspect of the caudate and lentiform nuclei.

The claustrum (Figs. 742,744) is a thin layer of gray substance, situated on the lateral surface of the external capsule. Its transverse section is triangular, with the apex directed upward. Its medial surface, contiguous to the external capsule, is smooth, but its lateral surface presents ridges and furrows corresponding with the gyri and sulci of the insula, with which it is in close relationship. The claustrum is regarded as a detached portion of the gray substance of the insula, from which it is separated by a layer of white fibers, the capsula extrema (band of Baillarger). Its cells are small and spindle-shaped, and contain yellow pigment; they are similar to those of the deepest layer of the cortex.

The nucleus amygdalæ (amygdala) (Fig. 741), is an ovoid gray mass, situated at the lower end of the roof of the inferior cornu. It is merely a localized thickening of the gray cortex, continuous with that of the uncus; in front it is continuous with the putamen, behind with the stria terminalis and the tail of the caudate nucleus.

The internal capsule (capsula interna) (Figs. 745,746) is a flattened band of white fibers, between the lentiform nucleus on the lateral side and the caudate nucleus and thalamus on the medial side. In horizontal section (Figs. 742) it is seen to be somewhat abruptly curved, with its convexity inward; the prominence of the curve is called the genu, and projects between the caudate nucleus and the thalamus. The portion in front of the genu is termed the frontal part, and separates the lentiform from the caudate nucleus; the portion behind the genu is the occipital part, and separates the lentiform nucleus from the thalamus.

The frontal part of the internal capsule contains: (1) fibers running from the thalamus to the frontal lobe; (2) fibers connecting the lentiform and caudate nuclei; (3) fibers connecting the cortex with the corpus striatum; and (4) fibers passing from the frontal lobe through the medial fifth of the base of the cerebral peduncle to the nuclei pontis. The fibers in the region of the genu are named the geniculate fibers; they originate in the motor part of the cerebral cortex, and, after passing downward through the base of the cerebral peduncle with the cerebrospinal fibers, undergo decussation and end in the motor nuclei of the cranial nerves of the opposite side. The anterior two-thirds of the occipital part of the internal capsule contains the cerebrospinal fibers, which arise in the motor area of the cerebral cortex and, passing downward through the middle three-fifths of the base of the cerebral peduncle, are continued into the pyramids of the medulla oblongata. The posterior third of the occipital part contains: (1) sensory fibers, largely derived from the thalamus, though some may be continued upward from the medial lemniscus; (2) the fibers of optic radiation, from the lower visual centers to the cortex of the occipital lobe; (3) acoustic fibers, from the lateral lemniscus to the temporal lobe; and (4) fibers which pass from the occipital and temporal lobes to the nuclei pontis.

The fibers of the internal capsule radiate widely as they pass to and from the various parts of the cerebral cortex, forming the corona radiata(Fig. 745) and intermingling with the fibers of the corpus callosum.

The external capsule (capsula externa) (Fig. 742) is a lamina of white substance, situated lateral to the lentiform nucleus, between it and the claustrum, and continuous with the internal capsule below and behind the lentiform nucleus. It probably contains fibers derived from the thalamus, the anterior commissure, and the subthalamic region.

The substantia innominata of Meynert is a stratum consisting partly of gray and partly of white substance, which lies below the anterior part of the thalamus and lentiform nucleus. It consists of three layers, superior, middle, and inferior. The superior layer is named the ansa lentiformis, and its fibers, derived from the medullary lamina of the lentiform nucleus, pass medially to end in the thalamus and subthalamic region, while others are said to end in the tegmentum and red nucleus. The middle layer consists of nerve cells and nerve fibers; fibers enter it from the parietal lobe through the external capsule, while others are said to connect it with the medial longitudinal fasciculus. The inferior layer forms the main part of the inferior stalk of the thalamus, and connects this body with the temporal lobe and the insula.

The stria terminalis (tænia semicircularis) is a narrow band of white substance situated in the depression between the caudate nucleus and the thalamus. Anteriorly, its fibers are partly continued into the column of the fornix; some, however, pass over the anterior commissure to the gray substance between the caudate nucleus and septum pellucidum, while others are said to enter the caudate nucleus. Posteriorly, it is continued into the roof of the inferior cornu of the lateral ventricle, at the extremity of which it enters the nucleus amygdalæ. Superficial to it is a large vein, the terminal vein (vein of the corpus striatum), which receives numerous tributaries from the corpus striatum and thalamus; it runs forward to the interventricular foramen and there joins with the vein of the choroid plexus to form the corresponding internal cerebral vein. On the surface of the terminal vein is a narrow white band, named the lamina affixa.

The Fornix (Figs. 720,747,748) is a longitudinal, arch-shaped lamella of white substance, situated below the corpus callosum, and continuous with it behind, but separated from it in front by the septum pellucidum. It may be described as consisting of two symmetrical bands, one for either hemisphere. The two portions are not united to each other in front and behind, but their central parts are joined together in the middle line. The anterior parts are called the columns of the fornix; the intermediate united portions, the body; and the posterior parts, the crura.

FIG. 746 Diagram of the tracts in the internal capsule. Motor tract red. The sensory tract (blue) is not direct, but formed of neurons receiving impulses from below in the thalamus and transmitting them to the cortex. The optic radiation (occipitothalamic) is shown in violet. (See enlarged image)

The body (corpus fornicis) of the fornix is triangular, narrow in front, and broad behind. The medial part of its upper surface is connected to the septum pellucidum in front and to the corpus callosum behind. The lateral portion of this surface forms part of the floor of the lateral ventricle, and is covered by the ventricular epithelium. Its lateral edge overlaps the choroid plexus, and is continuous with the epithelial covering of this structure. The under surface rests upon the tela chorioidea of the third ventricle, which separates it from the epithelial roof of that cavity, and from the medial portions of the upper surfaces of the thalami. Below, the lateral portions of the body of the fornix are joined by a thin triangular lamina, named the psalterium (lyra). This lamina contains some transverse fibers which connect the two hippocampi across the middle line and constitute the hippocampal commissure. Between the psalterium and the corpus callosum a horizontal cleft, the so-called ventricle of the fornix (ventricle of Verga), is sometimes found.

The columns (columna fornicis; anterior pillars; fornicolumns) of the fornix arch downward in front of the interventricular foramen and behind the anterior commissure, and each descends through the gray substance in the lateral wall of the third ventricle to the base of the brain, where it ends in the corpus mammillare. From the cells of the corpus mammillare the thalamomammillary fasciculus (bundle of Vicq dAzyr) takes origin and is prolonged into the anterior nucleus of the thalamus. The column of the fornix and the thalamomammillary fasciculus together form a loop resembling the figure 8, but the continuity of the loop is broken in the corpus mammillare. The column of the fornix is joined by the stria medullaris of the pineal body and by the superficial fibers of the stria terminalis, and is said to receive also fibers from the septum pellucidum. Zuckerkandl describes an olfactory fasciculus which becomes detached from the main portion of the column of the fornix, and passes downward in front of the anterior commissure to the base of the brain, where it divides into two bundles, one joining the medial stria of the olfactory tract; the other joins the subcallosal gyrus, and through it reaches the hippocampal gyrus.

FIG. 748 The fornix and corpus callosum from below. (From a specimen in the Department of Human Anatomy of the University of Oxford.) (See enlarged image)

The crura (crus fornicis; posterior pillars) of the fornix are prolonged backward from the body. They are flattened bands, and at their commencement are intimately connected with the under surface of the corpus callosum. Diverging from one another, each curves around the posterior end of the thalamus, and passes downward and forward into the inferior cornu of the lateral ventricle (Fig. 750). Here it lies along the concavity of the hippocampus, on the surface of which some of its fibers are spread out to form the alveus, while the remainder are continued as a narrow white band, the fimbria hippocampi, which is prolonged into the uncus of the hippocampal gyrus. The inner edge of the fimbria overlaps the fascia dentata hippocampi (dentate gyrus) (page 827), from which it is separated by the fimbriodentate fissure; from its lateral margin, which is thin and ragged, the ventricular epithelium is reflected over the choroid plexus as the latter projects into the chorioidal fissure.

Interventricular Foramen (foramen of Monro).Between the columns of the fornix and the anterior ends of the thalami, an oval aperture is present on either side: this is the interventricular foramen, and through it the lateral ventricles communicate with the third ventricle. Behind the epithelial lining of the foramen the choroid plexuses of the lateral ventricles are joined across the middle line.

The Anterior Commissure (precommissure) is a bundle of white fibers, connecting the two cerebral hemispheres across the middle line, and placed in front of the columns of the fornix. On sagittal section it is oval in shape, its long diameter being vertical and measuring about 5 mm. Its fibers can be traced lateralward and backward on either side beneath the corpus striatum into the substance of the temporal lobe. It serves in this way to connect the two temporal lobes, but it also contains decussating fibers from the olfactory tracts.

The Septum Pellucidum (septum lucidum) (Fig. 720) is a thin, vertically placed partition consisting of two laminæ, separated in the greater part of their extent by a narrow chink or interval, the cavity of the septum pellucidum. It is attached, above, to the under surface of the corpus callosum; below, to the anterior part of the fornix behind, and the reflected portion of the corpus callosum in front. It is triangular in form, broad in front and narrow behind; its inferior angle corresponds with the upper part of the anterior commissure. The lateral surface of each lamina is directed toward the body and anterior cornu of the lateral ventricle, and is covered by the ependyma of that cavity.

The cavity of the septum pellucidum (cavum septi pellucidi; pseudocele; fifth ventricle) is generally regarded as part of the longitudinal cerebral fissure, which has become shut off by the union of the hemispheres in the formation of the corpus callosum above and the fornix below. Each half of the septum therefore forms part of the medial wall of the hemisphere, and consists of a medial layer of gray substance, derived from that of the cortex, and a lateral layer of white substance continuous with that of the cerebral hemispheres. This cavity is not developed from the cavity of the cerebral vesicles, and never communicates with the ventricles of the brain.

The Choroid Plexus of the Lateral Ventricle (plexus chorioideus ventriculus lateralis; paraplexus) (Fig. 750) is a highly vascular, fringe-like process of pia mater, which projects into the ventricular cavity. The plexus, however, is everywhere covered by a layer of epithelium continuous with the epithelial lining of the ventricle. It extends from the interventricular foramen, where it is joined with the plexus of the opposite ventricle, to the end of the inferior cornu. The part in relation to the body of the ventricle forms the vascular fringed margin of a triangular process of pia mater, named the tela chorioidea of the third ventricle, and projects from under cover of the lateral edge of the fornix. It lies upon the upper surface of the thalamus, from which the epithelium is reflected over the plexus on to the edge of the fornix (Fig. 723). The portion in relation to the inferior cornu lies in the concavity of the hippocampus and overlaps the fimbria hippocampi: from the lateral edge of the fimbria the epithelium is reflected over the plexus on to the roof of the cornu (Fig. 749). It consists of minute and highly vascular villous processes, each with an afferent and an efferent vessel. The arteries of the plexus are: (a) the anterior choroidal, a branch of the internal carotid, which enters the plexus at the end of the inferior cornu; and (b) the posterior choroidal, one or two small branches of the posterior cerebral, which pass forward under the splenium. The veins of the choroid plexus unite to form a tortuous vein, which courses from behind forward to the interventricular foramen and there joins with the terminal vein to form the corresponding internal cerebral vein.

When the choroid plexus is pulled away, the continuity between its epithelial covering and the epithelial lining of the ventricle is severed, and a cleft-like space is produced. This is named the choroidal fissure; like the plexus, it extends from the interventricular foramen to the end of the inferior cornu. The upper part of the fissure, i.e., the part nearest the interventricular foramen is situated between the lateral edge of the fornix and the upper surface of the thalamus; farther back at the beginning of the inferior cornu it is between the commencement of the fimbria hippocampi and the posterior end of the thalamus, while in the inferior cornu it lies between the fimbria in the floor and the stria terminalis in the roof of the cornu.

The tela chorioidea of the third ventricle (tela chorioidea ventriculi tertii; velum interpositum) (Fig. 750) is a double fold of pia mater, triangular in shape, which lies beneath the fornix. The lateral portions of its lower surface rest upon the thalami, while its medial portion is in contact with the epithelial roof of the third ventricle. Its apex is situated at the interventricular foramen; its base corresponds with the splenium of the corpus callosum, and occupies the interval between that structure above and the corpora quadrigemina and pineal body below. This interval, together with the lower portions of the choroidal fissures, is sometimes spoken of as the transverse fissure of the brain. At its base the two layers of the velum separate from each other, and are continuous with the pia mater investing the brain in this region. Its lateral margins are modified to form the highly vascular choroid plexuses of the lateral ventricles. It is supplied by the anterior and posterior choroidal arteries already described, The veins of the tela chorioidea are named the internal cerebral veins (venæ Galeni); they are two in number, and run backward between its layers, each being formed at the interventricular foramen by the union of the terminal vein with the choroidal vein. The internal cerebral veins unite posteriorly in a single trunk, the great cerebral vein (vena magna Galeni), which passes backward beneath the splenium and ends in the straight sinus.

FIG. 750 Tela chorioidea of the third ventricle, and the choroid plexus of the left lateral ventricle, exposed from above. (See enlarged image)

Structure of the Cerebral Hemispheres.The cerebral hemispheres are composed of gray and white substance: the former covers their surface, and is termed the cortex; the latter occupies the interior of the hemispheres.

The white substance consists of medullated fibers, varying in size, and arranged in bundles separated by neuroglia. They may be divided, according to their course and connections, into three distinct systems. (1) Projection fibers connect the hemisphere with the lower parts of the brain and with the medulla spinalis. (2) Transverse or commissural fibers unite the two hemispheres. (3) Association fibers connect different structures in the same hemisphere; these are, in many instances, collateral branches of the projection fibers, but others are the axons of independent cells.

1. The projection fibers consist of efferent and afferent fibers uniting the cortex with the lower parts of the brain and with the medulla spinalis. The principal efferent strands are: (1) the motor tract, occupying the genu and anterior two-thirds of the occipital part of the internal capsule, and consisting of (a) the geniculate fibers, which decussate and end in the motor nuclei of the cranial nerves of the opposite side; and (b) the cerebrospinal fibers, which are prolonged through the pyramid of the medulla oblongata into the medulla spinalis: (2) the corticopontine fibers, ending in the nuclei pontis. The chief afferent fibers are: (1) those of the lemniscus which are not interrupted in the thalamus; (2) those of the superior cerebellar peduncle which are not interrupted in the red nucleus and thalamus; (3) numerous fibers arising within the thalamus, and passing through its stalks to the different parts of the cortex (page 810); (4) optic and acoustic fibers, the former passing to the occipital, the latter to the temporal lobe.

2. The transverse or commissural fibers connect the two hemispheres. They include: (a) the transverse fibers of the corpus callosum, (b) the anterior commissure, (c) the posterior commissure, and (d) the lyra or hippocampal commissure; they have already been described.

3. The association fibers(Fig. 751) unite different parts of the same hemisphere, and are of two kinds: (1) those connecting adjacent gyri, short association fibers; (2) those passing between more distant parts, long association fibers.

The long association fibers include the following: (a) the uncinate fasciculus; (b) the cingulum; (c) the superior longitudinal fasciculus; (d) the inferior longitudinal fasciculus; (e) the perpendicular fasciculus; (f) the occipitofrontal fasciculus; and (g) the fornix.

(b) The cingulum is a band of white matter contained within the cingulate gyrus. Beginning in front at the anterior perforated substance, it passes forward and upward parallel with the rostrum, winds around the genu, runs backward above the corpus callosum, turns around the splenium, and ends in the hippocampal gyrus.

(c) The superior longitudinal fasciculus passes backward from the frontal lobe above the lentiform nucleus and insula; some of its fibers end in the occipital lobe, and others curve downward and forward into the temporal lobe.

(f) The occipitofrontal fasciculus passes backward from the frontal lobe, along the lateral border of the caudate nucleus, and on the mesial aspect of the corona radiata; its fibers radiate in a fan-like manner and pass into the occipital and temporal lobes lateral to the posterior and inferior cornua. Déjerine regards the fibers of the tapetum as being derived from this fasciculus, and not from the corpus callosum.

(g) The fornix connects the hippocampal gyrus with the corpus mammillare and, by means of the thalamomammillary fasciculus, with the thalamus (see page 839). Through the fibers of the hippocampal commissure it probably also unites the opposite hippocampal gyri.

Structure of the Cerebral Cortex (Fig. 754).The cerebral cortex differs in thickness and structure in different parts of the hemisphere. It is thinner in the occipital region than in the anterior and posterior central gyri, and it is also much thinner at the bottom of the sulci than on the top of the gyri. Again, the minute structure of the anterior central differs from that of the posterior central gyrus, and areas possessing a specialized type of cortex can be mapped out in the occipital lobe.

On examining a section of the cortex with a lens, it is seen to consist of alternating white and gray layers thus disposed from the surface inward: (1) a thin layer of white substance; (2) a layer of gray substance; (3) a second white layer (outer band of Baillarger or band of Gennari); (4) a second gray layer; (5) a third white layer (inner band of Baillarger); (6) a third gray layer, which rests on the medullary substance of the gyrus.

Nerve Cells.According to Cajal, the nerve cells are arranged in four layers, named from the surface inward as follows: (1) the molecular layer, (2) the layer of small pyramidal cells, (3) the layer of large pyramidal cells, (4) the layer of polymorphous cells.

The Molecular Layer.In this layer the cells are polygonal, triangular, or fusiform in shape. Each polygonal cell gives off some four or five dendrites, while its axon may arise directly from the cell or from one of its dendrites. Each triangular cell gives off two or three dendrites, from one of which the axon arises. The fusiform cells are placed with their long axes parallel to the surface and are mostly bipolar, each pole being prolonged into a dendrite, which runs horizontally for some distance and furnishes ascending branches. Their axons, two or three in number, arise from the dendrites, and, like them, take a horizontal course, giving off numerous ascending collaterals. The distribution of the axons and dendrites of all three sets of cells is limited to the molecular layer.

The Layer of Small and the Layer of Large Pyramidal Cells.The cells in these two layers may be studied together, since, with the exception of the difference in size and the more superficial position of the smaller cells, they resemble each other. The average length of the small cells is from 10 to 15μ; that of the large cells from 20 to 30μ. The body of each cell is pyramidal in shape, its base being directed to the deeper parts and its apex toward the surface. It contains granular pigment, and stains deeply with ordinary reagents. The nucleus is of large size, and round or oval in shape. The base of the cell gives off the axis cylinder, and this runs into the central white substance, giving off collaterals in its course, and is distributed as a projection, commissural, or association fiber. The apical and basal parts of the cell give off dendrites; the apical dendrite is directed toward the surface, and ends in the molecular layer by dividing into numerous branches, all of which may be seen, when prepared by the silver or methylene-blue method, to be studded with projecting bristle-like processes. The largest pyramidal cells are found in the upper part of the anterior central gyrus and in the paracentral lobule; they are often arranged in groups or nests of from three to five, and are named the giant cells of Betz. In the former situation they may exceed 50μ in length and 40μ in breadth, while in the paracentral lobule they may attain a length of 65μ.

Layer of Polymorphous Cells.The cells in this layer, as their name implies, are very irregular in contour; they may be fusiform, oval, triangular, or star-shaped. Their dendrites are directed outward, but do not reach so far as the molecular layer; their axons pass into the subjacent white matter.

There are two other kinds of cells in the cerebral cortex. They are: (a) the cells of Golgi, the axons of which divide immediately after their origins into a large number of branches, which are directed toward the surface of the cortex; (b) the cells of Martinotti, which are chiefly found in the polymorphous layer; their dendrites are short, and may have an ascending or descending course, while their axons pass out into the molecular layer and form an extensive horizontal arborization.

Nerve Fibers.These fill up a large part of the intervals between the cells, and may be medullated or non-medullatedthe latter comprising the axons of the smallest pyramidal cells and the cells of Golgi. In their direction the fibers may be either tangential or radial. The tangential fibers run parallel to the surface of the hemisphere, intersecting the radial fibers at a right angle. They constitute several strata, of which the following are the more important: (1) a stratum of white fibers covering the superficial aspect of the molecular layer (plexus of Exner); (2) the band of Bechterew, in the outer part of the layer of small pyramidal cells; (3) the band of Gennari or external band of Baillarger, running through the layer of large pyramidal cells; (4) the internal band of Baillarger, between the layer of large pyramidal cells and the polymorphous layer; (5) the deep tangential fibers, in the lower part of the polymorphous layer. The tangential fibers consist of (a) the collaterals of the pyramidal and polymorphous cells and of the cells of Martinotti; (b) the branching axons of Golgis cells; (c) the collaterals and terminal arborizations of the projection, commissural, or association fibers. The radial fibers.Some of these, viz., the axons of the pyramidal and polymorphous cells, descend into the central white matter, while others, the terminations of the projection, commissural, or association fibers, ascend to end in the cortex. The axons of the cells of Martinotti are also ascending fibers.

FIG. 754 Cerebral cortex. (Poirier.) To the left, the groups of cells; to the right, the systems of fibers. Quite to the left of the figure a sensory nerve fiber is shown. (See enlarged image)

Special Types of Cerebral Cortex.It has been already pointed out that the minute structure of the cortex differs in different regions of the hemisphere; and A. W. Campbell126 has endeavored to prove, as the result of an exhaustive examination of a series of human and anthropoid brains, that there exists a direct correlation between physiological function and histological structure. The principal regions where the typical structure is departed from will now be referred to.

1. In the calcarine fissure and the gyri bounding it, the internal band of Baillarger is absent, while the band of Gennari is of considerable thickness, and forms a characteristic feature of this region of the cortex. If a section be examined microscopically, an additional layer of cells is seen to be interpolated between the molecular layer and the layer of small pyramidal cells. This extra layer consists of two or three strata of fusiform cells, the long axes of which are at right angles to the surface; each cell gives off two dendrites, external and internal, from the latter of which the axon arises and passes into the white central substance. In the layer of small pyramidal cells, fusiform cells, identical with the above, are seen, as well as ovoid or star-like cells with ascending axons (cells of Martinotti). This is the visual area of the cortex, and it has been shown by J. S. Bolton127 that in old-standing cases of optic atrophy the thickness of Gennaris band is reduced by nearly 50 per cent.

A. W. Campbell says: Histologically, two distinct types of cortex can be made out in the occipital lobe. The first of these coats the walls and bounding convolutions of the calcarine fissure, and is distinguished by the well-known line of Gennari or Vicq dAzyr; the second area forms an investing zone a centimetre or more broad around the first, and is characterized by a remarkable wealth of fibers, as well as by curious pyriform cells of large size richly stocked with chromophilic elementscells which seem to have escaped the observation of Ramón y Cajal, Bolton, and others who have worked at this region. As to the functions of these two regions there is abundant evidence, anatomical, embryological, and pathological, to show that the first or calcarine area is that to which visual sensations primarily pass, and we are gradually obtaining proof to the effect that the second investing area is constituted for the interpretation and further elaboration of these sensations. These areas therefore deserve the names visuo-sensory and visuo-psychic.

2. The anterior central gyrus is characterized by the presence of the giant cells of Betz and by a wealth of nerve fibers immeasurably superior to that of any other part (Campbell), and in these respects differs from the posterior central gyrus. These two gyri, together with the paracentral lobule, were long regarded as constituting the motor areas of the hemisphere; but Sherrington and Grunbaum have shown128 that in the chimpanzee the motor area never extends on to the free face of the posterior central gyrus, but occupies the entire length of the anterior central gyrus, and in most cases the greater part or the whole of its width. It extends into the depth of the central sulcus, occupying the anterior wall, and in some places the floor, and in some extending even into the deeper part of the posterior wall of the sulcus.

3. In the hippocampus the molecular layer is very thick and contains a large number of Golgi cells. It has been divided into three strata: (a) s. convolutum or s. granulosum, containing many tangential fibers; (b) s. lacunosum, presenting numerous vascular spaces; (c) s. radiatum, exhibiting a rich plexus of fibrils. The two layers of pyramidal cells are condensed into one, and the cells are mostly of large size. The axons of the cells in the polymorphous layer may run in an ascending, a descending, or a horizontal direction. Between the polymorphous layer and the ventricular ependyma is the white substance of the alveus.

5. The Olfactory Bulb.In many of the lower animals this contains a cavity which communicates through the olfactory tract with the lateral ventricle. In man the original cavity is filled up by neuroglia and its wall becomes thickened, but much more so on its ventral than on its dorsal aspect. Its dorsal part contains a small amount of gray and white substance, but it is scanty and ill-defined. A section through the ventral part (Fig. 755) shows it to consist of the following layers from without inward:

FIG. 756 Areas of localization on lateral surface of hemisphere. Motor area in red. Area of general sensations in blue. Auditory area in green. Visula area in yellow. The psychic portions are in lighter tints. (See enlarged image)

1. A layer of olfactory nerve fibers, which are the non-medullated axons prolonged from the olfactory cells of the nasal cavity, and reach the bulb by passing through the cribriform plate of the ethmoid bone. At first they cover the bulb, and then penetrate it to end by forming synapses with the dendrites of the mitral cells, presently to be described.

2. Glomerular Layer.This contains numerous spheroidal reticulated enlargements, termed glomeruli, produced by the branching and arborization of the processes of the olfactory nerve fibres with the descending dendrites of the mitral cells.

3. Molecular Layer.This is formed of a matrix of neuroglia, imbedded in which are the mitral cells. These cells are pyramidal in shape, and the basal part of each gives off a thick dendrite which descends into the glomerular layer, where it arborizes as indicated above, and others which interlace with similar dendrites of neighboring mitral cells. The axons pass through the next layer into the white matter of the bulb, and after becoming bent on themselves at a right angle, are continued into the olfactory tract.

4. Nerve Fiber Layer.This lies next the central core of neuroglia, and its fibers consist of the axons or afferent processes of the mitral cells passing to the brain; some efferent fibers are, however, also present, and end in the molecular layer, but nothing is known as to their exact origin.

Weight of the Encephalon.The average weight of the brain, in the adult male, is about 1380 gms.; that of the female, about 1250 gms. In the male, the maximum weight out of 278 cases was 1840 gms. and the minimum weight 964 gms. The maximum weight of the adult female brain, out of 191 cases, was 1585 gms. and the minimum weight 879 gms. The brain increases rapidly during the first four years of life, and reaches its maximum weight by about the twentieth year. As age advances, the brain decreases slowly in weight; in old age the decrease takes place more rapidly, to the extent of about 28 gms.

The human brain is heavier than that of any of the lower animals, except the elephant and whale. The brain of the former weighs from 3.5 to 5.4 kilogm., and that of a whale, in a specimen 19 metres long, weighed rather more than 6.7 kilogm.

Cerebral Localization.Physiological and pathological research have now gone far to prove that a considerable part of the surface of the brain may be mapped out into a series of more or less definite areas, each of which is intimately connected with some well-defined function.

Motor Areas.The motor area occupies the anterior central and frontal gyri and the paracentral lobule. The centers for the lower limb are located on the uppermost part of the anterior central gyrus and its continuation on to the paracentral lobule; those for the trunk are on the upper portion, and those for the upper limb on the middle portion of the anterior central gyrus. The facial centers are situated on the lower part of the anterior central gyrus, those for the tongue, larynx, muscles of mastication, and pharynx on the frontal operculum, while those for the head and neck occupy the posterior end of the middle frontal gyrus.

FIG. 757 Areas of localization on medial surface of hemisphere. Motor area in red. Area of general sensations in blue. Visual area in yellow. Olfactory area in purple. The psychic portions are in lighter tints. (See enlarged image)

Sensory Areas.Tactile and temperature senses are located on the posterior central gyrus, while the sense of form and solidity is on the superior parietal lobule and precuneus. With regard to the special senses, the area for the sense of taste is probably related to the uncus and hippocampal gyrus. The auditory area occupies the middle third of the superior temporal gyrus and the adjacent gyri in the lateral fissure; the visual area, the calcarine fissure and cuneus; the olfactory area, the rhinencephalon. As special centers of much importance may be noted: the emissive center for speech on the left inferior frontal and anterior central gyri (Broca); the auditory receptive center on the transverse and superior temporal gyri, and the visual receptive center on the lingual gyrus and cuneus.